1. Field of the Invention
The present invention is directed to flexible multiply towpreg and a method of production therefor and, more specifically, a very flexible multiply towpreg suitable for weaving, braiding, and the like, and a method for producing such a towpreg.
2. Discussion of the Prior Art
Towpreg consists of two components, continuous fibers or filaments combined with a matrix resin. Conventional towpreg consists of hundreds or thousands of the fibers or filaments embedded in a continuous mass of the matrix. The advanced reinforcing fibers typically used are available commercially in continuous form in "bundles" of filaments known as "tows" which vary widely in number of filaments per tow. Many matrix resins are available, however two kinds of matrix resin systems dominate the prior art: thermoset and thermoplastic polymers.
Thermoplastic polymers have been widely used as matrices for composites, and are potentially useful as matrices for advanced composites for aerospace applications. Thermoplastics have advantages over thermosetting materials in fracture toughness, impact strength and environmental resistance. Thermoplastics also provide prepregs with indefinite shelf life, give the fabricator better quality assurance and avoids the storage and refrigeration problem associated with thermosetting prepreg. Thermoplastic molecules are tougher than the rigid cross-linked network of the thermosets; few of the toughened thermosets have met the combined requirements of damage tolerance and hot/wet compression strength necessary for use in aerospace composites.
The disadvantage of thermoplastic polymers as a composite matrix material is the difficulty of uniformly coating the fibers due to the high viscosity of the molten polymer. Also the poor wetting characteristics of carbon fibers, when used as the tow, leads to the formation of voids. Additionally, thermoplastic towpreg is rigid and is not well-suited for weaving or braiding as the resulting fabrics are stiff. Similarly, the rigidity of thermoplastic towpreg complicates the filament winding of complex shapes; heat must be focused at the point of contact to achieve towpreg conformability during layup.
Thermoset polymers also are used as matrices for towpreg. Towpreg containing thermosetting prepolymer, although relatively flexible, is tacky, thus requiring a protective release coating which must be removed prior to use. While thermoset towpreg is acceptable for filament winding, its tackiness and the requirement of a protective release coating make thermoset towpreg unfeasible for weaving, braiding, or the production of chopped fiber feed stock for bulk or sheet molding compounds.
Two other types of towpreg are: metal matrix towpreg, which is similar to thermoplastic towpreg, and partially cured ceramic towpreg which is similar to thermoset towpreg. However, due to the limited markets for metal and ceramic composites, these materials are not generally available and have not been the focus of developments in manufacturing technology. Metal and ceramic matrix towpregs can be manufactured in a manner and form analogous to thermoplastic and thermosetting towpregs.
Continuous fiber tow prepregs can be produced by a number of impregnation methods including hot melt, solution, emulsion, slurry, surface polymerization, fiber commingling, film interleaving, electroplating, and dry powder techniques.
In hot melt processing, impregnation may be accomplished by forcing the fiber and resin through a die at high temperature under conditions that create high shear rates. Due to the high temperatures of this process, the thermoplastic material can degrade. Other disadvantages of this process include the high stress applied to the tow and difficulties in impregnating the tow with thermoplastics.
In solution coating, the matrix material is dissolved in solvent and the fiber tow is passed through this solution and then dried to evaporate the solvent. Two disadvantages associated with this process is that thermoplastics usually exhibit limited solubility at high concentration, and most engineering thermoplastics cannot be dissolved in a low boiling solvent at room temperature. Additionally, high solution viscosity results in the same impregnation problems as with hot melt, as well as causing the fiber to stick together. Another problem is the difficulty in removing the solvent. Further, traces of solvent left in the towpreg lead to undesirable porosity in the composites.
An emulsion process is one way to apply powdered polymer matrix material with a very small particle size to fiber tow by synthesizing the resin as an aqueous emulsion with a surfactant. The problem with this process is that the removal of the surfactant from the final composite is difficult.
Slurry coating or wet powder processing is a non-solvent coating technique designed to resolve the problem of the insolubility of most thermoplastics in a solvent at room temperature. In slurry coating, the powder is suspended in a liquid medium, generally water, wherein no solvency exists between the resin and the medium, and the fiber bundles are drawn through the slurry. The slurry particulate matrix does not wetout the fiber, resulting in the need for higher pressures to consolidate the matrix and fibers into a composite. This towpreg can be tacky which is not suitable for weaving or braiding. Other disadvantages include the necessity for the removal of the liquid medium, volatiles, and dispersants or surfactants which are used to form the polymer/liquid colloidal state, the likelihood of aggregates in the slurry caused by poor mixing, and the possibility that polymer particles will settle during processing.
To achieve intimate mixing in emulsion or slurry coating, the particle size of the slurry or emulsion should be smaller than the fiber diameter. For many of the thermoplastics that cannot be made by emulsion or dispersion polymerization, it is extremely difficult to produce such fine powder. Thus, a coarse blend between fibers and particles is obtained. The quality of the blend decreases as the particle size increases, leading to poor matrix distribution in the consolidated composite.
In surface polymerization, coating results from the polymerization of the polymer matrix on the fiber substrate. Disadvantages associated with this process include the need to remove undesired materials, such as solvent, inhibitor or by-products of the polymerization reactions. Surface polymerization can be only conducted by special techniques, such as electropolymerization, which cannot be used to produce many of the polymeric matrices of commercial interest.
In fiber commingling, the polymeric matrix is introduced in fibrous form. Polymeric and reinforcing fibers are mingled as dry blends; however, wetting of the fiber by a process such as melting the polymer fiber is deterred until the composite is consolidated. Effective impregnation depends on the degree of randomness of the intermingling of the resin fiber throughout the system. Since no wetting of the reinforcing fibers by the matrix material occurs in this tow, higher pressures are needed to consolidate the towpreg under equivalent processing times and temperatures into a composite, as compared to completely wetted towpreg.
Film casting is one method for producing towpreg which resolves some of the problems associated with hot melt impregnation of thermoplastics. It consists of stacking a film layer of matrix material cast from either hot melt or solution over spread tow. The fibers sandwiched between two films are heated and calendered to force the resin into the fibers.
A flexible metal matrix towpreg has been produced by electroplating by American Cyanamid. Each fiber is completely wetout and uniformly coated with the metal matrix. This towpreg has desirable properties for weaving, braiding, filament winding, or for conversion to bulk or sheet molding compound feedstocks. Additionally, this towpreg does not require high pressure for consolidation. However, electroplating has a disadvantage in that systems which can be adapted to electroplating are limited. Most ceramic and polymeric matrices of commercial interest cannot be electroplated on reinforcing fibers. Since the filaments or fibers comprise one of the electrodes, the reinforcing fiber must be conductive and fibers such as glass fibers and organic fibers cannot be electroplated.
Powder coating of tow is the most recent method developed in prepreg tow technology. The significant advantages are that no solvent is required and no high stress is introduced in the process. The ultimate goal for almost all powder coating applications is the ability to deposit a thin, even thickness, high quality coating as efficiently as possible. The powder resin also must be solid at ambient and elevated storage temperatures, and be capable of melting sharply to low viscosity to permit flow and to penetrate the fiber tow when heated.
Dry powder coating has many advantages because the elimination of a wet base, solvent or water, facilitates reclamation of the coating material. This is an important economic advantage which promises a potential 100% utilization of powder plus elminination of expensive solvents that are flushed off and inevitably wasted. Thus, powder coating was conceived and has grown largely on these potential benefits.
Another method of powder coating tow with matrix is the ATOCHEM method of impregnating tow fibers with dry thermoplastic polymer powder and then covering the towpreg with a thermoplastic sheath. However, as the matrix is not melted onto the fibers, only fibers in contact with the sheath are wetted with the matrix. Consolidating this composite also requires higher pressures under comparable processing conditions than completely wetout towpreg. Furthermore, as in slurry coating, a fine polymer powder, generally less than 20 microns, is recommended for this process. Producing such fine powder from thermoplastics can be very expensive.